|Kanji||神宮寺 誠一郎 (じんぐうじ せいいちろう)|
|Relatives||Unnamed Father (deceased)|
Renge (mother, deceased)
Jinguji Ren (younger brother)
|Anime Debut||Season 2: Episode 6|
|Seiyuu||Ueki Makoto (植木 誠)|
Seiichiro Jinguji (神宮寺 誠一郎, Jinguuji Seiichirou) is Ren Jinguji's older brother and the president of the Jinguji Group. He is voiced by Makoto Ueki (植木 誠, Ueki Makoto).
His hair is rather in an orange-toned color that is between dark and light. He has blue colored eyes. He wears a blue jacket and pants with eyeglasses.
In the game, he is very strict and tight to Ren. He also seems to be very selfish because he wants Ren to be an idol so that he can advertise their company but deep inside he wanted Ren to know what he wants in life. He can also be very polite and kind when it comes to the people around like Haruka.
He is the one responsible for making Ren go to Saotome Academy so that he can become an idol for the Jinguji Group Commercials. He is not introduced properly in the first season of the anime, and only appears in Ren's flashback in episode four. He is then properly introduced in the second season of the anime; in episode six where the Jinguji Financial Group is the sponsor of the Japan Boy's Collection, Ren has been invited to feature in it. At the end of the episode, he and Ren have a heart-to-heart talk where Seiichiro reveals that it was not their parents who supported him with his work, but Ren's singing. He explains to Ren that the reason why he made him go to Saotome Academy was not that he wanted to use him as a poster boy, but it was because he loved Ren's songs and wanted him to make people happy with the songs he sings.
- His name means "honest first son".